Various suggestions have been made for removing biological tissue with the aid of photodecomposition (photoetching) by means of short laser pulses. Publications directed to this subject matter are listed below.
1. Health Physics Vol. 40, 1981, p. 677 to 683, Taboda et al: "Response of the corneal Epithelium to KrF Excimer Laser Pulses";
2. American Journal of Ophthalmology 96, 1983 p. 710 to 715, Trokel et al: "Excimer surgery of the cornea";
3. Ophthalmology 92, 1985, p. 741 to 748, Puliafito et al: "Excimer laser ablation of the cornea and lens";
4. Arch. Ophthalmology 103, 1985, p. 1741/1742, Krueger and Trokel: "Quantitation of corneal ablation by ultraviolet laser light";
5. Ophthalmology 92, 1985, p. 749 to 758, Marshall et al: "An ultrastructural study of corneal incisions induced by an excimer laser at 193 nm";
6. American Journal of Ophthalmology Vol. 103, p. 713 and 714, Berlin et al: "Excimer laser Photoablation in Glaucoma Filtering Surgery";
7. American Journal of Ophthalmology Vol. 99, p. 483 and 484, Pellin et al: "Endoexcimer laser Intraocular Ablative Photodecomposition";
8. Arch. Ophthalmology Vol. 104, 1986, p. 1825 to 1829 Nanevicz: "Excimer laser ablation of the lens".
Tissue removal with pulsed lasers at the eye is also the subject matter of U.S. Pat. Nos. 4,686,979 and 4,744,360.
The above publications and patents describe how corneal tissue, lenticular nuclei, scleral tissue in the iridocorneal angle, vitreous body tissue or tissue of the ocular fundus can be removed pursuant to the method of photoremoval with minimal thermal necrosis of the edges of the incision while utilizing excimer lasers at wavelengths of 193 nm and 308 nm. Finer incisions are possible with excimer lasers in this wavelength range than with Nd-YAG or CO.sub.2 -lasers in the wavelength range between 1 .mu.m and 10 .mu.m. This is so not only because of the shorter wavelengths of the excimer laser, but also because of the basically different process of photodecomposition which is a threshold process as described in the publications and takes place only above a predetermined energy density in dependence upon the effective wavelength of the laser used.
When performing surgery on tissue by means of laser radiation, the difficulty always occurs to select the wavelength of the laser radiation so that the wavelength corresponds to the absorption characteristics of the tissue. If this match is not provided, then the energy density of the laser radiation required for tissue removal is to be selected high and then a relatively large thermal necrosis zone occurs along the incision edges.
Because of these difficulties, the tendency is to utilize different lasers for surgical incisions at different biological tissues or to operate at different wavelengths. This however requires a substantial effort.